1. © Professor Alan Hedge, Cornell University, November 2002
Lifting and Back Stress
DEA325/651
Professor Alan Hedge
Low Back Pain
Low back pain occurs in 80% of adults at some point in their lives
Low back pain is second only to upper respiratory infections as a cause for
absence from work.
Back symptoms are among the 10 leading reasons for patient visits to
emergency rooms, hospital outpatient departments, and physicians' offices.
Low back pain often recurs, 12 and in 5- 10% of patients low back pain
becomes chronic.
Back symptoms are the most common cause of disability for persons under
age 45.
Low Back Pain
Low back pain costs approximately $24 billion per year in direct medical
expenses and another $27 billion per year in lost productivity and
compensation. Total annual costs for back pain increase from $35 to $56
billion when disability costs are included.
Almost 90% of all cases of low back pain are due to sprains and strains and by
definition almost always improve.
Low back pain from herniation of a disk affects only a small percentage of
workers.
The Human Spine
The Flexible Spine
Lumbar spine and dorsal nerves
Vertebrae, Spinal Cord and Nerve Roots
Structure of the Vertebral Unit
Lumbar disc structure
Intervertebral disc structure
Nucleus Pulposus
NP allows for the different planes of
intervertebralmovement.
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2. © Professor Alan Hedge, Cornell University, November 2002
Intervertebral discs
Vertebrae differ in size in different regions of the spine.
Intervertebral discs
NP position and size differs in different regions of the
spine.
Lumbar Disc Motion
Intervertebral disc forces
Compressive forces exert the major influence on low back
injury risks.
Spinal forces
Flexion-Extension - asymmetric compression of the
intervertebral disc from forwards/backwards bending movements.
Spinal forces
Lateral bending - asymmetric compression of the intervertebral
disc.
Spinal forces
Axial rotation - torsional deformation of the intervertebral disc.
Spinal forces
Shear - displacement of one vertebra over the over.
Spinal forces
Tension - stretching (elongation) of the spine results in
elongation of the intervertebral discs.
Spinal disc forces
Discs do not have a blood supply and they rely on pressure changes for the
transport of nutrients and waste products.
Posture and Lumbar Disc Pressure
(Wilke et al., 1999, Spine, 24, 755)
Lumbar intradiscal pressure can be recorded at L4-L5 during
different standing, sitting and lifting postures.
Posture and Lumbar Disc Pressure
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3. © Professor Alan Hedge, Cornell University, November 2002
(Wilke et al., 1999, Spine, 24, 755)
Spinal Shrinkage - Stadiometer
Stature and Time-of-Day
Over a day the spine can shrink ~ 22mm (0.9”)
Lumbar disc damage
Lifting Posture
Many types of industrial task require lifting objects.
Lifting is often done in a poor posture.
Lifting Injuries
Incidence rates represent the number of injuries and/or illnesses
per 100 full-time workers and are calculated as:
(N/EH) X 200,000 where:
N = number of injuries and/or illnesses
EH = total hours worked by all employees during the calendar year
200,000 = base for 100 full-time equivalent workers (working 40 hours per
week, 50 weeks per year).
NIOSH Lifting Equation (1981)
Action Limit (AL) = 90 lbs. X (6/H)(1-0.1[V-30])(.7+3/D)(1-F/Fmax)
Where:
H = horizontal location (inches) forward of he midpoint of the ankles at the origin of the lift to
the mid-point of the hand grasps (load center)
V = vertical location (inches) of the hands above the floor at the start of the lift
D = vertical travel distance (inches) between origin and destination
F = average frequency of lifts (lifts/minute)
Fmax = maximum frequency which can be sustained (from table).
Maximum Permissible Limit (MPL) = 3(AL)
NIOSH Lifting Equation (1991)
Recommended Weight Limit (RWL) = LC x HM x VM x DM x AM x FM x CM
Where:
LC = load constant (51 lbs.)
HM = horizontal multiplier = 10/H
VM= vertical multiplier = (1-(0.0075[V-30])
DM = distance multiplier= (0.82+1.8/D)
AM = asymmetric multiplier = (1-(0.0032A))
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4. © Professor Alan Hedge, Cornell University, November 2002
FM = frequency multiplier (from table)
CM = coupling multiplier (from table)
A = angle of asymmetry = angular displacement of the load from the
mid-sagittal plane
{H,V,D, F same as 1981 equation}
NIOSH Lifting Equation (1991)
Lifting Index (LI)
The LI is a term that provides a relative estimate of the level of physical stress
associated with a particular manual lifting task. The estimate of the level of
physical stress is defined by the relationship of the weight of the load lifted
and the recommended weight limit. The LI is defined by the following
equation:
LI = Load Weight = L
Recommended Weight Limit RWL
LI <1 is the goal for all lifting tasks
LI >1 poses an increased risk for lifting-related low back pain and greater % of workers at
risk
LI >3 poses an increased risk for work-related injury
NIOSH Lifting Equation (1991)
Single task job assessment – a job where the lifting variables do
not significantly change from task to task.
Multi-task job assessment – a job where there are signficant
differences in the different lifting tasks, and each task is assessed
separately.
The Frequency-Independent Recommended Weight Limit (FIRWL) and
Single-Task Recommended Weight Limit (STWRL) are calculated for each
task.
The Frequency-Independent Lifting Index (FILI) and Single-Task Lifting
Index (STLI) are computed for each task.
Composite lifting index (CLI) then computed for the overall job based on
individual analyses.
NIOSH Lifting Equation (1991)
NIOSH Calculator
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5. © Professor Alan Hedge, Cornell University, November 2002
Source: http://www.humantech.com
NIOSH Lifting Equation (1991)
RWL defined for an 8 hours work period.
RWL set to protect 99% of men and 75% of women workers (in reality it’s closer to 95% of
men and 85% of women).
Assumes that lifting task is two-handed, smooth, in front of the body, hands are at the same
height or level, moderate-width loads (i.e., <= the body width of the lifter), load evenly
distributed between both hands.
Other manual handling activities are minimal and don’t require significant energy expenditure
(i.e., holding, pushing, pulling, carrying, walking or climbing).
Temperatures (66-79°F) or humidity (35-50%). Outside of the ranges may increase the risk of
injury.
http://www.cdc.gov/niosh/ergopage.html (manual for NIOSH equation)
http://www.industrialhygiene.com/calc/lift.html (on-line calculator for NIOSH equation)
NIOSH Lifting Equation (1991)
Equation doesn’t apply to one-handed lifts, lifting while seated or kneeling,
lifting in a constrained or restricted work space, lifting unstable loads,
wheelbarrows or shoveling tasks.
Assumes that the shoe sole to floor surface coupling provides for firm footing.
Assumes the same level of risk for low back injuries from lifting or lowering.
Underestimates the lifting hazard when situations don't conform to
assumptions.
http://www.safetyinfo.com/medical/lifting-ana-expl.htm (good overview of
equation).
Snook Tables
(Snook S. and Ciriello V., 1991. The design of manual handling tasks: revised tables of maximum
acceptable weights and forces. Ergonomics, 1197-1213.)
Weight limit tables developed by Stover Snook at the Liberty Mutual Insurance
Research Center.
Tables give weight limit values based on:
Box width - distance from the front of the body
Vertical lift distance
Percentage of the industrial population covered by the limit
Knuckle height
Lift frequency
Separate data tables for men and women
Separate data tables for lifting, lowering, pushing, pulling and carrying
Snook tables
Mital Tables
(Mital A., Nicholson A., and Ayoub M., 1993. A guide to manual materials handling. Taylor &
Francis, London.)
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6. © Professor Alan Hedge, Cornell University, November 2002
Mital tables used to get the maximum acceptable weight (MAW) for a task, at a specific
population percentiles. Uses the same population and database as Snook tables.
Values adjusted for various biomechanical, physiological, and epidemiological criteria.
Tables also account for seven factors to modify the maximum acceptable weight to simulate a
realistic material handling tasks:
Working duration;
Limited headroom (spatial restraint);
Asymmetrical lifting;
Load asymmetry;
Couplings (grip characteristic);
Load placement clearance; and
Heat stress.
Other Lifting Solutions
Drop the load and roll it.
Other Lifting Solutions
Make the load too heavy for a manual lift and use lifting
equipment.
Other Lifting Solutions
Make the load too heavy for a manual lift and use lifting
equipment.
Additional Resources
NIOSH Lifting Equation (1991)
ErgoEaser
Source: http://nattie.eh.doe.gov/others/ergoeaser/download.html
NIOSH Lifting Equation (1991)
NIOSH RWL Calculator
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7. © Professor Alan Hedge, Cornell University, November 2002
Source: http://ekginc.com/nioshcalc.htm
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